I can’t even begin to count the number of times I encounter a new metal clay artist struggling with the concept of firing. It’s not surprising, considering that almost every formula of metal clay lists a different firing schedule. The most recent formulas even promise results via torch firing, which is fantastic, but adds greatly to the chaos. 1290 for 10 minutes? 1110 for 30? Which one is right?
The problem is that to be able to choose the right firing schedule you have to understand the concept of sintering, and unfortunately most metal clay books and classes don’t even mention it.
Sintering, simply put, is the process by which your metal fuses under heat. When you fire a piece of metal clay, the first few minutes are devoted just to heating it up. Around 800 degrees, the binder in the clay combusts and you’ve got a small blaze on your hands (which is particularly exciting when you’re torch-firing your piece). Once the binder is completely burned away, the rest of the heating time is devoted to fusing, or sintering, your metal.
Think of the particles of metal in your metal clay piece as ice cubes in a freezer. At first, they’re all just jumbled up inside the ice box, right? But the colder the freezer and the longer they sit inside it, the more they begin to fuse together. The little spaces of air between them gradually get smaller and smaller as the ice cubes become a dense mass. Eventually, you’ve got to take the whole chunk out and whack at it with a hammer in order to get manageable ice cubes again.
Your metal is doing the same thing as a reaction to the heat of firing. The tiny metal particles are gradually growing closer together and the mass as a whole is becoming denser. And that, in a nutshell, is sintering. (As a side note, the more recent formulas of metal clay utilize a lot of varied shapes and sizes of metal particles rather than uniform ones, so they fit together better and can more easily fill in those air pockets. Neat!)
The important thing to take away from all of this is that the more sintered your piece is, the stronger it will be (just like that glacier you found in your freezer). If you only fire a piece at the minimum temperature and time it will be metal, but it will also be very brittle. In order to get maximum strength, you should fire your piece at the maximum temperature for the maximum amount of time (in the case of PMC3, 1650 for 2 hours). You should also never include ramp time in your firing time – let your kiln heat fully to temperature before timing your firing.
Sometimes you’ll have to lower that temperature and/or that time to deal with various inclusions, such as gemstones or sterling silver components, that can’t handle the heat. This is just fine, so long as you keep in mind the trade-off for including those things is sacrificing a little bit of strength. You want to be especially careful with items, like rings, that will be getting banged about a lot.
Torch-firing is especially risky. Personally, I avoid torch firing whenever possible, but each artist has to make that decision for themselves, and I know several who torch fire exclusively with great success. Just be warned: if you plan on doing any work to your piece after firing (dapping, texturing, that sort of thing), then torch-firing won’t cut it – your piece will break.
If you have pieces you fired for a shorter time, though, don’t worry! You can always re-fire for full strength at any time. Just be aware that the longer you fire a piece, the more it will shrink. If you torch fired a ring and now want to kiln fire it for the full 2 hours at 1650, be prepared to lose at least a full ring size to shrinkage.
Forward, and happy claying!